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Many athletes are concerned with attaining or maintaining an optimal body weight and body composition for their sport. Athletes may want to reduce body weight to ensure optimal performance, to improve aesthetic appearance, or to compete in weight category sports. This leads to efforts to reduce body fat without losing muscle mass and often to nutritional practices that may have severe health consequences. Is there an optimal strategy for reducing body weight without compromising health or performance? Such a strategy has been proposed by Paoli et al. in the present issue of this Journal (6).

The authors claim that a ketogenic diet can be used by athletes to produce weight loss without impairing performance, especially strength performance. A ketogenic diet is high in fat and low in carbohydrates (≤50 g d-1) and fairly high in protein. There are numerous randomized controlled studies showing that ketogenic diets effectively reduce body fat without causing excessive loss of lean body tissue (3). The crucial question is if — or how — these diets influence sports performance. Many nutrition-exercise manipulations have been studied in an effort to increase rates of fatty acid oxidation and attenuate the rate of carbohydrate utilization, thus potentially augmenting exercise performance.

The evidence suggests that an increased fat availability transfers into higher rates of both whole-body and muscle lipid utilization during standardized submaximal aerobic exercise. However, despite greater rates of fat oxidation, these diets consistently fail to improve endurance performance compared with a carbohydrate-rich diet (2,4) and we know little about the effect of a ketogenic diet on strength performance. One study in 21-year-old gymnasts who followed a ketogenic diet for 30 days revealed a reduction in fat mass of 1.9 kg, with no changes in either muscle mass or strength performance (7). These findings suggest that a ketogenic diet may be an effective strategy for weight loss while maintaining muscle mass and performance, but a strategy that also improves performance would be optimal.

Do we need to adopt a ketogenic diet when there are well-documented diets that produce a safe weight loss without eliminating most carbohydrate from the diet, a feature that may make it difficult to restore muscle glycogen stores and sustain the diet? The Diogenes randomized controlled trial with obese subjects showed that a slight reduction in glycemic load by reducing total carbohydrate intake and focusing on low glycemic index foods, and slightly increased protein content, produced a safe weight loss with a reduction in cardiometabolic risk factors (5), and the diet was considered easy and palatable (1). These dietary principles, which to a larger extent satisfy carbohydrate requirements and decrease fat intake, are more relevant for athletes than a strict ketogenic diet.

Paoli et al. suggest that the ketogenic diets in sports should be explored further. However, the rationale for this seems questionable (2).